Organic field effect transistors (OFETs) are used in display devices and logic capable circuits. Different metals have been used as the source/drain electrodes in the organic field effect transistors. A widely used electrode material is gold (Au), however, its high cost and disadvantageous processing properties have shifted the focus to possible alternatives like for example Ag, Al, Cr, Ni, Cu, Pd, Pt, Ni or Ti. Copper (Cu) is one of the possible alternative electrode materials for Au, as it has a high conductivity, a relatively low price and is easier for the usual manufacturing processes. In addition, copper is already used in the semiconductor industry, therefore it is easier to switch the large scale production process of electronic devices to organic semiconductor materials as a new technology, when combined with the already established copper technology for the electrodes.
However, when using copper as the electrode, i.e. as charge carrier injection metal, there is a disadvantage due to its low work function, which is below the level of most modern organic semiconductors.
DE 10 2005 005 089 A1 describes an OFET comprising copper source and drain electrodes which are surface modified by providing a copper oxide layer thereon. However, since the copper in an ambient atmosphere tends to oxidize to Cu2O and then to CuO and further to Cu hydroxides, this can create a non-metal conductive layer on the Cu electrode which results in limited charge carrier injection into the semiconductor layer.
In prior art there are known methods of metal or metal oxide electrode modification in order to improve charge carrier injection, which are based e.g. on thiol compounds.
For example, US 2008/0315191 A1 discloses an organic TFT comprising source and drain electrodes formed of a metal oxide, wherein the electrode surfaces are subjected to surface treatment by applying a thin film, with a thickness of 0.3 to 1 molecular layer, of a thiol compound, for example pentafluorobenzenethiol, perfluoroalkylthiol, trifluoromethanethiol, pentafluoroenthanethiol, heptafluoropropanethiol, nonafluorobutanethiol, sodium butanethiol, sodium butanoate thiol, sodium butanol thiol or aminothiophenol. However this approach is effective mainly for gold electrodes, but not for copper electrodes because, compared to a gold surface, on a copper surface the thiol groups form weaker chemical bonds.
It is therefore an aim of the present invention to provide improved methods for modifying metal or metal oxide electrodes or charge injection layers, including but not limited to copper electrodes, in organic electronic devices, in order to overcome the drawbacks of metal electrodes known from prior art, like low work function and low oxidative stability. Another aim is to provide improved electrodes and charge injection layers based on metal or metal oxides for use in organic electronic devices, in particular OFETs and OLEDs, and methods for their preparation. Another aim is to provide improved organic electronic devices, in particular OFETs and OLEDs, and methods for their preparation, containing a modified metal or metal oxide electrode according to the present invention. The methods, electrodes and devices should not have the drawbacks of prior art methods and allow time-, cost- and material-effective production of electronic devices at large scale. Other aims of the present invention are immediately evident to the expert from the following detailed description.
It was found that these aims can be achieved by providing processes for electrode treatment, materials used in such processes, electrodes treated by such processes, and devices containing such treated electrodes as described in the present invention. In particular, the present invention is related to a chemistry-based treatment process for metal electrodes which improves their work function and their properties of charge carrier injection into an organic semiconductor. This is achieved by providing a process for subjecting the electrode surface to a self-assembled monolayer (SAM) treatment process with a chemical class of compounds known as benzotriazoles (BTA), or derivatives or structural analogues of these compounds, which are optionally substituted with electron withdrawing groups, like e.g. F or CN, and/or surface active groups, like e.g. thiol or perfluoroalkyl groups. It was found that this is a very efficient method of electrode modification, especially when applied to copper electrodes, even in the presence of copper oxides, which improves the work function of the electrode and thereby improves its charge carrier injection into semiconductor layer. The surface treatment process according to the present invention enables the manufacture of electronic devices, in particular of OFETs, with improved source/drain electrodes.
Benzotriazoles are known in prior art as pharmaceutical compounds, and have also been proposed for use as passivation materials in the inorganic semiconductor industry, mainly for protection in chemical-mechanical polishing processes, as described for example in “Review on copper chemical-mechanical polishing (CMP) and post-CMP cleaning in ultra large system integrated (ULSI)—An electrochemical perspective”, E-E. Yair and Starosvetsky D., Electrochimica Acta, 52, 2007, 1825. However, they have hitherto not been suggested for SAM treatment to improve the work function of metal electrodes in organic electronic devices.
US 2009/0121192 A1 discloses a method for enhancing the corrosion resistance of an article comprising an Ag coating which is deposited on a solderable Cu substrate. This is achieved by exposing the Ag coating to an anti-corrosion composition comprising a multifunctional molecule, wherein said multifunctional molecule comprises at least one nitrogen-containing organic functional group that interacts with and protects Cu surfaces, and further comprises at least one sulphur-containing organic functional group that interacts with and protects Ag surfaces. However, whereas the method has the aim to enhance of the corrosion resistance of the Ag coating, there is no hint or suggestion to a method for changing the properties of the metal with the aim to improve its charge carrier injection when used as electrode in an organic electronic device.